Substrate metalization has been widely used in microelectronics and optoelectronics industries. It is often necessary to apply one or more inner metal layers in addition to the desired top metal layer to a substrate to form a functional metalized structure. The metalized structures may consist of two or more dissimilar metals due to various requirements such as adhesion, diffusion barrier, thermal barrier, oxidation resistance, and so on. Such multi-layer metalized structures have a galvanic corrosion problem when exposed to moisture, owing to the differences in electrochemical potentials of the dissimilar metals. Galvanic corrosion cells are created at areas of water condensation during device service. Such galvanic corrosion can cause current leakage, short circuit, or other problems causing device failures.
Many electronic and electo-optic devices include electrically conductive electrodes formed on the substrate material. Gold (Au) is commonly used as an electrode material because it exhibits high electrical conductivity and excellent resistance to chemical corrosion. Copper (Cu), silver, and platinum (Pt) are also used as electrode materials. These materials, however, do not readily adhere to typical substrate materials. For example, gold does not easily adhere to lithium niobate (LiNbO3), which is an electro-optic substrate material that is used in devices such as light modulators for optical communication systems.
Accordingly, adhesion layers are frequently deposited on the substrate prior to depositing the electrodes to improve the adhesion. Known adhesion layers are thin film layers of an active metal or metal alloy that have good adhesion to a desired substrate and good adhesion to gold or other electrode metals. For example, chromium (Cr), titanium (Ti), or titanium-tungsten (TiW) adhesion layers have been used to form gold electrodes on lithium niobate substrates.
The electrodes are typically deposited on the adhesion layer by electroplating. The adhesion layer between the gold electrodes is then etched away, thereby leaving completed electrodes that form electrical contact to the device. Unfortunately, electrodes formed with known methods of fabricating gold electrodes with an adhesion layer are sometimes unreliable.
Galvanic corrosion caused by the presence of dissimilar metals can occur around the electrode-adhesion layer interface when the device is exposed to moisture and ionic contamination. The active metal adhesion layer functions as an anode and corrodes in the presence of moisture and condensation inside the device package. Corrosion negatively impacts the performance and reduces the service life of an electronic or electro-optic device.
Another problem with known adhesion layers is that the final etching step to remove the adhesion layer between the electrodes often undercuts the adhesion layer beneath the electrode. The undercut can negatively affect the performance of electronic or electro-optic devices. The undercut can also trap moisture and contaminants and thus increase galvanic corrosion.
Galvanic corrosion can be reduced by reducing the amount of moisture around the electrode-active metal interface. This is sometimes difficult or impossible to accomplish if the device is not hermetically sealed. Galvanic corrosion can even occur in hermetically sealed devices in some circumstances. For example, galvanic corrosion can occur if moisture is present during the hermetic sealing process, if out-gassing occurs from packaging materials, or if leaks occur in the package after it is sealed.
To overcome the above-mentioned problem, an electrode structure including a gold-encapsulated adhesion layer was proposed in U.S. Pat. Ser. No. 10/261,877 filed on Oct. 1, 2002 to X. Chen. However, while the encapsulated adhesion layer mitigates the galvanic corrosion problem, it also introduces additional manufacturing steps in the electrode fabrication process.
It is an object of the instant invention to provide a metalized structure and a method of forming the metalized structure that reduces and/or eliminates the above-mentioned corrosion problems.
It is a further object of the instant invention to provide an electrode structure and a method of forming the electrode structure that obviates the need for an adhesion layer.